Gas springs are well known and have been used in dies of presses for sheet metal stamping operations. Conventional gas springs have a gas chamber which receives a pressurized gas that provides a force on a piston and a solid piston rod which carries the piston to bias them to an extended position. The pressurized gas resists the movement of the piston and the piston rod from their extended position to a retracted position. Various housings and seals are provided in the gas spring to retain the piston and piston rod within a casing of the gas spring and to prevent leakage of the pressurized gas from the gas chamber.
The solid piston rod takes up significant volume in the gas spring and a conventional design has a relatively small diameter piston rod compared to the internal diameter of a casing in which the piston rod reciprocates. Because the ratio of piston rod diameter to casing diameter is small, the pressure increase during piston rod travel is less than 100% although the force curve is not very flat, as generally shown by line A in FIG. 10.
To gain more effective force, piston rods have been made larger in diameter. To avoid an undesired pressure increase during a stroke due to use of the larger diameter piston rod, the piston rod can be made to some degree hollow. However, as the depth of the blind bore increases, the cost to manufacture the piston rod increases dramatically. To achieve a longer stroke, a longer bearing surface is needed. But a longer bearing surface requires a longer piston rod, which in turn requires a deeper and much more costly bore. Further, the longer bearing assembly consumes gas chamber volume and thereby leads to a greater pressure increase during the stroke. The force increase during a piston rod stroke is higher in this type design, as shown by line B in FIG. 10.